1. Technical Field
The present invention relates in general to integrated circuits and in particular to an operating environment monitor circuit providing a signal which can be utilized for compensating application circuits. Still more particularly, the present invention relates to a temperature variation monitor which can provide a reference signal responsive to temperature variations in the operating environment.
2. Description of the Related Art
A primary challenge in designing precision integrated circuits is to control circuit parameters, such as bias currents, in view of temperature variations. During the design of integrated circuits, anticipating and controlling operating fluctuations due to changes in environmental conditions such as temperature, requires a complicated analysis. Temperature sensitivity, fabrication variations and supply voltage variations exhibit complex relationships among each other. In integrated circuits, precision oscillators and more particularly, oscillators with feedback, such as phase lock loop topologies, are designed to comply with precise specifications that have little margin for deviation. Surmounting these tight tolerances over a wide range of temperatures is very challenging and difficult for a designer. Today's integrated circuits require higher accuracy and tighter tolerances due to their lower voltage and higher frequency requirements.
Techniques for stabilization of circuits over the temperature range which the circuit will endure, have received a substantial amount of attention recently. Examples of precision circuits, or circuits which are very sensitive to temperature variations include voltage controlled oscillators, phase lock loops, and switch mode power supply controllers.
Known compensation techniques for circuits requiring accurate oscillations are extensive and diverse. Compensation techniques are typically external to the application circuit. Prior art circuits compensate for temperature variations utilizing indirect methods. Generally, indirect methods sense the deviation of the actual output from the desired output. Indirect methods do not specifically sense the change in operating parameters of the devices which deviate and cause undesirable complications in the output signal. Indirect methods many times utilize resistors and diodes which are external to the semiconductor chip. Hence, compensation is not performed utilizing detection of the source of the problem, but compensation is performed by sensing the adverse effects, which the temperature or voltage variation has created on the desired signal.
Timing problems associated with environmental variations such as temperature are encountered frequently in both digital and analog circuits. Temperature variation can significantly alter the performance characteristics of CMOS circuits, and many times, temperature variations limit system performance.
Circuits which are sensitive to temperature variations, such as voltage-controlled oscillators, can be significantly improved by temperature-independent biasing techniques. However, in many applications this technique is not sufficient to meet the performance requirements. Techniques involving temperature sensing and compensation are complex and usually involve very large gain amplifiers to convert the small sensor signals to larger usable levels.
Other known compensation systems utilize a digital memory and a digital-to-analog converter to generate a reference current for compensation. In these designs, the reference current is utilized to stabilize operation over a given temperature range.
Tighter process tolerances can be adopted to improve device performance and reduce device variability, but tighter tolerances substantially increases the cost of the product. Testing and selection of temperature insensitive devices to isolate acceptable devices is also utilized in the prior art. Selection also increases the cost of the product due to additional testing effort and the corresponding lower yield.
A typical temperature compensation circuit compensates for operating variations utilizing elements in a feedback circuit. Most known temperature compensation circuits are based upon sensor devices to provide feedback which is correlated to the desired operating output of the application circuit or circuit to be compensated.
Known temperature compensation circuits adjust for circuit variations by subsidizing a circuit's output. Hence, present temperature compensation techniques are deficient because they do not monitor actual device parameters, such as transconductance, which drift during temperature variations.
To compensate for temperature variations, complex arrangements have hitherto been required. Hence, a need exists for a simple effective and efficient temperature variation detector or monitor which can be utilized to compensate for operating aberrations due to temperature variations.